Recently, headlamp covers for an automobile are flattened and curved in order to achieve a good style and reduce air resistance. Thus, materials for headlamp covers are being switched from glass which is hard to process to plastics having easy moldability. In particular, a polycarbonate resin is preferably used among plastic materials because of its excellent thermal resistance against high temperature due to the lamp sealed in the headlamp cover for an automobile. Such a requirement for a polycarbonate resin exists also in other plastics, and various plastics are widely used in place of glass, wood and metals in many products. In the following part, a polycarbonate resin is mainly described.
The polycarbonate resin has an excellent thermal resistance but low weatherability by its nature, and it is liable to strongly discolor in yellow or deteriorate. The scratch resistance and chemical resistance are also poor. Therefore, it is difficult to use the polycarbonate resin for the above-mentioned headlamp covers for an automobile, and the like, without any protection.
As a method to solve such problems, there is provided a method to coat coating such as an urethane containing a UV absorbent on the surface of a polycarbonate molded product and thermally cure to form a protective film. This method can improve the scratch resistance but sometimes fails in obtaining sufficient hardness and weatherability.
On the other hand, recently, various methods to use a UV curing resin as a protective coating have been proposed. In this case, two methods are adopted in order to avoid the disadvantage that a UV curing resin which contains a UV absorbent is cured with UV ray, (1) a double coating method and (2) a thickening by coating method.
The double coating method is a method to form a coated film of non-UV curing type containing a UV absorber, then to form a UV curing resin film of good scratch resistance. This method, however, is easy to balance performances by dividing the functions, but has problems that the process become complex and delamination is liable to occur in the interface between two coated layers.
The thickly coating method is a method to make the coating as thick as possible so that the concentration per unit weight of the UV absorbent can be kept low, to thereby prevent the retardation in UV curing and form a coating with a good appearance. In this method, however, cracks tend to occur with the passage of time on the coated film due to the thick coating. Softer coating to avoid the problem results in reduction of scratch resistance.
Prevention of the deterioration due to UV and improvement of scratch resistance of products are effective from the standpoint of keeping the product appearance and extending the product life for polycarbonate resin as well as for other plastics. Accordingly, it is strongly desired to develop polycarbonate resin molded products having good scratch resistance, weatherability, and a coated surface with a good appearance, wherein a coating is coated which meets the resin properties of polycarbonate resin as the substrate and has good adhesion, scratch resistance and appearance of coated surface as well as excellent weatherability.